Dueling in the medieval period was a regulated part of the legal system that had certain requirements in how they must be conducted. I have a few previous responses which are relevant here.

This one looks at a specific type of judicial duel, that over property rights in England

This one focuses on on the decline of it looking at what is generally pointed to as the last judicial duel, fought between Jarnac and La Châtaigneraye in 1547, and highlights the process by which they were granted the field.

I'd also point here which is focused less on the judicial duel than the development of the duel of honor - which was illegal - but begins by looking at the transitional period from grant of the field at the tail end, such as above, to illicit activity, a transition that moved from medieval style to what we see in the early modern period.